The present invention relates to vehicle bumper beams, and more particularly relates to a bumper beam having a front section of continuous shape and a back section attached to the front section to make a tubular beam of changing cross-sectional size.
Two basic types of bumper beams often used on modern vehicles are tubular sections (also called closed sections, such as “B” or “D” shapes) and open sections (such as “C” sections or “hat” sections). The tubular sections and also the open sections each have their own advantages and disadvantages. For example, from an engineering standpoint, bumper beams made from tubular sections are inherently more rigid and capable of absorbing and/or transmitting more energy (especially based on a strength-to-weight ratio) on impact due to the way that impact stresses are distributed around and along the tubular shapes. In contrast, open sections tend to prematurely buckle during impact since the “legs” of the open sections will spread apart, kink, and quickly lose shape upon impact. However, open sections tend to allow more styling and product variation. There is a concurrent strong desire to use high-strength materials for bumpers because it reduces weight while providing higher impact strengths (as compared to lower strength materials). However as higher and higher-strength materials are used, it becomes more and more difficult to form raw sheet stock into the desired beam shape, because the higher-strength materials are harder and harder on tooling and the presses that run them. This is especially true for stamping presses and stamping dies, where the dies move perpendicularly against a sheet to form the sheet. Roll-forming processes have the ability to form higher-strength materials than stamping processes, however roll-forming processes are limited to producing a constant cross-sectional shape along a length of the roll-formed parts.
Roll-forming is a particularly attractive manufacturing method because dimensionally-accurate bumper beams can be mass-produced at good production speeds, with minimal manual labor, and using high-strength materials, yet the tooling can be made more durable and long-lasting than stamping dies when used to shape ultra-high-strength steels and high-strength low-alloy steels. For example, Sturrus U.S. Pat. No. 5,092,512 and Sturrus U.S. Pat. No. 5,454,504 disclose roll-forming apparatus of interest. However, as noted above, a drawback to roll-forming is that the roll-forming process can only produce a constant cross section over the entire length of the part. Further, the material thickness and material strength of the raw coil stock cannot change around a given cross section, since the material begins as a unitary coil of material. Regarding the constant cross section produced by roll-forming, this often does not satisfy current styling trends which require variations in cross-sectional size along a length of the beam due to packaging space over the vehicle frame rails (versus the packaging space available at a centerline of the vehicle), or which require a longitudinal sweep with an increased curvature at corners of the vehicle (e.g. at the fenders). These styling conditions require roll-formed tubular parts to be end-formed or taper cut at their ends by secondary processes. But these secondary processes are expensive because end-forming and/or taper cutting the parts is not easy (particularly when they are made of high-strength materials). Also, the process of end-forming and/or taper cutting require more than one secondary process. For example, taper cutting requires some sort of cap to cover the sharp edges that result from the cutting process, which must be accurately fixtured and then welded in place. Alternatively, the ends of the tubular sections may be reformed to better fit functional and aesthetic styling concerns (see Sturrus U.S. Pat. No. 5,306,058), but it is difficult to accurately and consistently deform the ends, thus potentially leading to unacceptable dimensional variations and high tooling wearout.
Beams made from C-shaped open sections can be formed to a desired three-dimensional shape, including non-uniform cross sections along their length, but their open section is inherently not as strong as a tubular shape during an impact. Specifically, the open sections include rearwardly-extending legs that tend to prematurely spread apart or otherwise collapse upon impact. This greatly reduces the beam's overall sectional impact strength and reduces its ability to consistently and predictably absorb energy. By stabilizing the legs of the front section, the front sections can be made much stronger and more energy-absorbing. This is sometimes done in prior art by adding reinforcements such as bulk heads, flat plating, and/or bridging between the legs to prevent the legs from prematurely spreading during an impact. (See FIG. 1 in the present drawings.) By stabilizing the legs of an open section, it can be made to come closer to matching the performance of the tubular sections. However, these additional reinforcements require expensive secondary operations since they utilize considerable fixturing and welding machinery, and they often require several additional parts and considerable assembly time and in-process inventory. Also, the process of welding multiple reinforcements to an open beam can be difficult to control, since multiple parts must be carefully separately fixtured and each and every one of the parts welded very consistently in place. Also, the location of each stabilizing strap can greatly affect impact strengths along the beam.
To summarize, packaging and performance requirements of bumper beams on vehicles and related vehicle front end (or rear end) components often increase the complexity of a bumper design since they result in the addition of other structural components, which might include bridges, bulkheads, radiator supports, fascia supports, fascia, and the like. Or they may require end treatment of the bumper beam to include end-forming or taper cutting, so as to form an increased angle at an end of the bumper in front of the fenders. This increase in complexity results in an increase in cost due to substantial secondary processing. It also results in difficult tradeoffs between function and styling criteria. It is desirable to provide a design and process that overcomes the drawbacks of constant cross section roll-formed sections, yet that takes advantage of roll-forming processes as a way of forming ultra-high-strength materials for use in bumper beams, as discussed below. Also, it is desirable to provide design flexibility that allows tuning of the bumper beam in the bumper development program, which can be very important for timing and investment reasons. At the same time, it is desirable that the ultra-high-strength steels be an option for components so that the bumper beam can be designed for optimally high strength-to-weight ratios. Still further, even though ultra-high-strength steels are used, it is desired that the arrangement allow for some use of less expensive materials and of materials that allow the use of relatively simple forming and bending tooling to minimize investment while still being able to form the ultra-high-strength materials without expensive tooling and without having tooling quickly wear out. In other words, it is desirable to utilize stamped or molded reinforcing components where possible and in combination with high-strength materials where it makes practical sense to do so.
An additional problem is that ultra-high-strength materials are difficult to form in stamping presses, or at least it is preferable not to do so. Specifically, those skilled in the art prefer not to stamp materials such as ultra-high-strength steels (UHSS) because the UHSS material is so strong that it is hard to form without cracking and that it damages or quickly wears out the stamping dies and the stamping press.
Thus, a bumper beam having the aforementioned advantages and solving the aforementioned problems is desired.